Hierarchical cellular communications systems employ different size cells to provide both wide-coverage, basic-service (macro cell) and high-quality, high-capacity radio coverage in smaller areas (micro cell). Micro cells are useful in specific areas where increased capacity is needed. For example, micro cells may be located to serve areas of concentrated traffic within the macro cell or to provide a high data rate service. A micro cell uses a low-height antenna and a low base station transmit power which result in a small cell and a short frequency reuse distance, thereby increasing capacity. Additional benefits of a smaller cell include a longer talk-time (battery life time) for the user since mobile stations will likely use a substantially lower uplink transmit power to communicate with a micro cell base station than with a base station in a larger macro cell which is likely farther away.
In a hierarchical cell structure (HCS), macro cells and micro cells typically overlap to handle different traffic patterns or radio environments. The micro cell base station may be connected to the macro cell base station via digital transmission lines, or the micro cell base station may be treated just like a macro cell and be connected directly to a base station controller node like a base station controller (BSC) in the well-known global system for mobile communications (GSM) systems or a radio network controller (RNC) in the third generation, wideband, code division multiple access (WCDMA) systems.
Smooth handover between macro and micro cells is important to provide continuous communication capability for mobile communications. But handover between macro cells and micro cells is more difficult to perform than traditional handover between macro cells, particularly in the case of large obstructions such as buildings often encountered in micro cell environments in the case of high velocity mobiles, and because of overlapping coverage areas. One way to perform HCS handover is to have the macro and micro cells employ different frequencies. The same cell class/size uses the same carrier frequency, and different cell classes/sizes use different carrier frequencies. Although CDMA enables efficient use of the same frequency channels in adjacent cells of the same hierarchical layer, (i.e., all micro cells), different frequency channels are assigned to cells on different hierarchical layers in order to avoid power control problems and excessive interference. But because of these different frequencies, “soft” handover and macro diversity are not feasible between macro and micro cells. Instead, an inter-frequency “hard” handover is required. Furthermore, seamless inter-frequency handovers are troublesome in CDMA systems. In addition to these disadvantages, using different frequencies limits efficient use of the available frequency spectrum simply because that spectrum must be divided between the different class/size cells. Twice as much bandwidth is needed if different frequencies are used. Moreover, if a guard zone must be maintained around the micro cell to protect it from a larger, more powerful, overlying macro cell, capacity is further reduced.
Alternatively, the macro and micro cells may employ the same frequency which allows soft handover and macro diversity to be used and provides more capacity. There are several well known advantages to soft handover in mobile communications including make-before-break handover to sustain continuous service at cell boundaries and compensation for fading and other disturbances to the received signal. And all systems hope to increase capacity.
For a same-frequency HCS system, however, a “balanced link” is conventionally believed necessary. A balanced link means that the cell boundary for uplink communications from a mobile station to a base station is the same as that for downlink communications from the base station to that mobile station. This means that for a mobile located at the cell boundary between macro and micro base stations, the same transmit power is required for the mobile station uplink signal to be received at the macro and micro base stations. In the downlink, the mobile receives a pilot signal from the macro and micro base stations at the same power level.
In order to balance the uplink and downlink, the two (or more) base stations adjust a pilot signal power allocation ratio—a ratio of pilot power to total transmission power per carrier frequency. Calculation of the pilot signal power allocation ratio is described in “Microcell Engineering in CDMA Cellular Networks,” by Shapira, IEEE Transactions on Vehicular Technology, Vol. 43, No. 4, November 1994, which is incorporated herein by reference. If the up and down links are unbalanced, then one link limits capacity while the other link has some capacity margin.
Transmit power control (TPC) is also required when the same frequency is used in mutual cells and in both the uplink and downlink directions, respectively. The purpose of transmit power control is to keep (1) the actual signal quality of mobile signals received at a base station close to a target signal quality and (2) the actual signal quality of base station signals received at a mobile station close to a target signal quality. But TPC is only effective for managing the power level of active communications between mobiles and the cell base station. TPC does not control the transmit power of a mobile communicating with another-cell in the uplink, and TPC does not control the power of a base station that is not communicating with the mobile sending the TPC command. For downlink communications in an HCS, fast fading and/or shadowing in the radio path may result in excessive interference to a mobile station within the micro cell. This is because the macro base station usually has a taller antenna and a greater total transmission power which may exceed the dynamic power control range of the micro base station. Transmit power control in the micro cell usually does not prevent this excessive interference from the macro base station to a mobile communicating with a micro base station. The excessive interference caused by the macro cell in the downlink results in lower capacity or even dropped calls in the micro cell.
Another problem in HCS systems is the premature and often unnecessary handover of a connection where the mobile station is moving at a high velocity. A fast moving mobile will not likely remain in the micro cell for enough time to warrant handover. It would be helpful to prevent unnecessary micro cell handovers for fast moving mobiles in HCS systems to avoid the overhead, signaling, and the resulting reduction in bandwidth associated with that signaling for unnecessary handovers.
The present invention solves these and other problems in the context of a HCS by using an unbalanced link. Macro and micro cells in the HCS use the same frequency/frequency band to increase capacity and attain diversity benefits as compared to using different frequencies/frequency bands. Complicated and time-consuming inter-frequency handover, which may sometimes be unstable, is also avoided. The micro cell downlink coverage is reduced so that the radio uplink and downlink for the HCS are purposefully unbalanced. A smaller micro cell in the downlink may be beneficial in terms of providing stable service to a highly-loaded but relatively-small service area.
The downlink micro cell coverage reduction may be accomplished in one example embodiment by actually reducing the micro cell pilot transmit power level. In another example embodiment, downlink micro cell coverage reduction may be accomplished by tilting an antenna beam of the micro cell base station that transmits/receives downlink/uplink signals from/to the micro cell. In a third example embodiment, downlink micro cell coverage reduction may be accomplished by mathematically reducing a detected power level of the micro cell pilot signal and using that reduced pilot power in handover decisions.
Handovers to the micro cell in an unbalanced link, being based on detected or otherwise determined micro cell pilot powers, occur less/later than if a balanced link was used. Indeed, some potential handovers will not even occur. Although the micro cell downlink coverage is reduced with an unbalanced link, mobiles that would be served by the micro cell with a balanced link are adequately served by the macro cell. And an unbalanced link offers more stable HCS service in the micro cell and provides greater overall capacity gain for the HCS system. Moreover, the unbalanced link offers the micro cell base station a downlink transmission power “margin.” Reduced coverage of the micro cell base station means that the total transmission power from the micro cell base station is reduced. The base station transmission power to a relatively close mobile station is relatively small compared with that to a mobile station relatively far from the base station. As a result, the micro cell base station has some additional margin/flexibility to increase its maximum transmit power if needed to reach a mobile station suffering excessive interference with acceptable signal power and quality.
A hierarchical cell structure (HCS) cellular communications system includes a macro cell encompassing a smaller micro cell that employ the same frequency band. The macro cell includes a macro cell base station, and the micro cell includes a micro cell base station. An uplink communication cell boundary between the macro cell and the micro cell is established, and a downlink communication cell boundary between the macro cell and the micro cell is established. A radio network controller determines whether a condition exists in the HCS system which indicates that the uplink and downlink micro cell boundaries should be unbalanced. If the condition is met or exists, the effective range of a downlink transmission from the micro cell base station is reduced —directly or indirectly—to unbalance the uplink and downlink HCS cell boundaries.
Three, non-limiting, example techniques for implementing an unbalanced link are described. First, the radio network may directly instruct the micro cell base station to reduce the power level of its pilot signal. Second, the radio network may instruct the micro cell base station to “tilt” the antenna corresponding to the micro cell pilot to decrease its azimuth angle thereby effectively decreasing the range of the pilot signal. Third, the radio network controller may send an offset value to one or more mobile stations near the micro cell. Those mobiles use that offset to mathematically reduce the detected pilot power from the micro base station. Because the received micro cell pilot power is decreased by that offset, the mobile perceives the micro cell base station as farther away, and is thus more likely to be serviced by the macro base station. Alternatively, the radio network may selectively apply the offset to mobile reported pilot powers to achieve a similar effect. These latter two “indirect” approaches may be desirable because they do not actually reduce the pilot power level. A strong pilot signal is necessary for accurate channel estimation and signal demodulation. But these two approaches still effect a reduced downlink micro cell size.
Another inventive feature relates to uplink interference resulting when mobiles near the micro cell transmit in an unbalanced link situation. Inter-cell interference will mainly be caused by a mobile transmitting near the micro cell which was forced to communicate with the macro base station because of the unbalancing. If the interference associated with an uplink transmission from the mobile station to the macro cell base station is likely to exceed a predetermined limit, then interference cancellation (IC) is preferably performed at a receiver in the micro cell base station.
Yet another inventive feature relates to high velocity mobile stations. If the speed of the mobile station exceeds a threshold, the radio network controller may prevent handover of the mobile connection to the micro cell by reducing (or reducing by a larger amount) the downlink range of the micro cell. A smaller micro cell downlink reduces the chance that a fast moving mobile will be handed over to the micro cell and then quickly handed right back to the macro cell. Overhead, signaling, and associated bandwidth reduction for the macro-to-micro-to-macro cell handovers are avoided.